Particle detection systems are used or applied in many facilities, such as electron beam system, scanning electron microscope (SEM), focused ion beam (FIB), mass spectrograph, or other facility necessary to detect particles with or without charge, photons.
Electron beam wafer inspection tool (EBWIT) needs large range of beam current to meet both high throughput (HT) mode and high resolution (HR) mode, in which the HT mode is driven by productivity and the HR mode is driven, on the opposite side, by design rule which now continues shrinking. The EBWIT usually uses several tens to several hundreds nA (nano Ampere) beam current for HT mode and uses several to several tens of pA (pico Ampere) beam current for HR mode. This requires the electron detector used in the EBWIT can handle the signal beam current (SE+BSE) from several tens of pA to several hundreds of nA. This requirement is a huge challenge for any current electron detection system.
Nowadays, there is no EBWIT that can be operated under both HT mode and HR mode; basically, HT mode is more popular due to the majority of the EBWIT detectors is SPD (semiconductor photodiode detector). For the HR mode, received detected current from specimen is too small for the SPD that output signal current, with about 2000 gain from the input signal current, is still a long way off to handle for the EBWIT.
Currently, semiconductor photo diode (SPD) is widely used for the EBWIT due to its low noise and potential of handling large beam current nature, such as several mA (mini Ampere). However, the gain obtained from SPD is very low (˜2000@10 KV) for pA detection current and not easy to be increased.
High gain, such as 104-107, can be acquired from using Phosphor-PMT (phosphor multiplier tube) system and MCP (micro channel plate) detectors. However, most PMT and MCP can only output maximal several uA signal current, which cannot meet the requirement of large beam current at low noise level, because the dynode in PMT limits the output current and the gain variation is too large when PMT dynode operates at low voltage. Further, although the Phosphor-PMT system can achieve high gain, but high bandwidth scintillator usually contributes high level white noise. However, if the image-average technique is used to eliminate the white noise, the throughput will be seriously hurt.
According to the above discussion, no detector system can output signal in the range from pA to mA. Thus, it is an important topic to formulate a particle detection system that can have a large range of gain and output signal current with low noise to meet both HT and HR modes requirement.
Further, in order to handle such a situation, Wang et al. suggested a Bi-Type detection system in U.S. patent application Ser. No. 12/715,766 filed on Mar. 2, 2010.